Contemporary longitudinally-pumped laser diode-pumped lasers have relied on laser diode pump sources with the limitation that the diodes are generally low power devices. Therefore, the laser output power has been limited by the power available from the laser diodes employed. While incoherent laser diodes of high power are available, these have been found to be less satisfactory since they are less efficient for longitudinal pumping than the coherent diodes. Although coherent diodes are available in powers of one to three watts each, combining more than two such diodes is generally complicated and inefficient.
A single gain-element ring laser is disclosed in the article entitled "A Single Frequency Nd:YAG Ring Laser Pumped by Laser Diodes" by Richard Scheps et al., IEEE Journal of Quantum Electronics, Vol. 26, No. 3, Mar. 1990. This design uses two single stripe one-watt laser diodes which form a single beam that is focused on the exterior face of a Nd:YAG rod. Single-longitudinal-mode performance for CW operation and single-transverse-multi-longitudinal-mode output performance for Q-switched operation were demonstrated in a combination of compact size and high efficiency. A design restraint on the use of the single gain-element ring laser is that its diode-pumped output power was limited by the available highest-power laser diodes which were appropriate for pumping. Combining a number of these diodes to provide for a higher output power was found to be not only cumbersome and generally inefficient, but, in addition, created a more serious impediment to the upward scaling of the output power. The heat deposited in the rod by pumping harder (using more laser diodes) puts an upper limit on how much pump power the rod can take and, hence, its upward scaling is impeded.
Thus, a continuing need exists in the state of the art for a diode pumped laser configuration having the capability for increased output powers that can be efficiently optically pumped.